Java parser: add support for unnamed classes (JEP-445)

Fixes #18584

compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala

@@ -826,6 +826,27 @@ object JavaParsers {
       addCompanionObject(statics, cls)
     }
 
+    def unnamedClassDecl(priorTypes: List[Tree], start: Offset): List[Tree] = {
+      val name = source.name.replaceAll("\\.java$", "").nn.toTypeName
+      val (statics, body) = typeBodyDecls(CLASS, name, Nil)
+
+      val (priorStatics, priorBody) = priorTypes.partition {
+        case t: TypeDef => t.mods.is(Flags.JavaStatic)
+        case _: ModuleDef => true
+        case _ => false
+      }
+
+      val cls = atSpan(start, 0) {
+        TypeDef(name, makeTemplate(
+          parents = Nil,
+          stats = priorBody ::: body,
+          tparams = Nil, 
+          needsDummyConstr = true)
+        ).withMods(Modifiers(Flags.Private | Flags.Final))
+      }
+      addCompanionObject(priorStatics ::: statics, cls)
+    }
+
     def recordDecl(start: Offset, mods: Modifiers): List[Tree] =
       accept(RECORD)
       val nameOffset = in.offset
@@ -1067,16 +1088,37 @@ object JavaParsers {
       val buf = new ListBuffer[Tree]
       while (in.token == IMPORT)
         buf ++= importDecl()
+
+      val afterImports = in.offset
+      val typesBuf = new ListBuffer[Tree]
+
       while (in.token != EOF && in.token != RBRACE) {
         while (in.token == SEMI) in.nextToken()
         if (in.token != EOF) {
           val start = in.offset
           val mods = modifiers(inInterface = false)
           adaptRecordIdentifier() // needed for typeDecl
-          buf ++= typeDecl(start, mods)
+
+          in.token match {
+            case ENUM | INTERFACE | AT | CLASS | RECORD => typesBuf ++= typeDecl(start, mods)
+            case _ =>
+              if (thisPackageName == tpnme.EMPTY_PACKAGE) {
+                // upon encountering non-types directly at a compilation unit level in an unnamed package,
+                // the entire compilation unit is treated as a JEP-445 unnamed class
+                //TODO support @annotated members of unnamed class
+                val cls = unnamedClassDecl(priorTypes = typesBuf.toList, start = afterImports)
+                typesBuf.clear()
+                typesBuf ++= cls
+              } else {
+                in.nextToken()
+                syntaxError(em"illegal start of type declaration", skipIt = true)
+                List(errorTypeTree)
+              }
+          }
         }
       }
-      val unit = atSpan(start) { PackageDef(pkg, buf.toList) }
+
+      val unit = atSpan(start) { PackageDef(pkg, (buf ++ typesBuf).toList) }
       accept(EOF)
       unit match
         case PackageDef(Ident(nme.EMPTY_PACKAGE), Nil) => EmptyTree

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -50,6 +50,9 @@ class CompilationTests {
     if scala.util.Properties.isJavaAtLeast("16") then
       tests ::= compileFilesInDir("tests/pos-java16+", defaultOptions.and("-Ysafe-init"))
 
+    if scala.util.Properties.isJavaAtLeast("21") then
+      tests ::= compileFilesInDir("tests/pos-java21+", defaultOptions.withJavacOnlyOptions("--enable-preview", "--release", "21"))  
+
     aggregateTests(tests*).checkCompile()
   }
 

compiler/test/dotty/tools/dotc/parsing/JavaJep445ParserTest.scala

@@ -0,0 +1,82 @@
+package dotty.tools.dotc.parsing
+
+import dotty.tools.DottyTest
+import dotty.tools.dotc.ast.Trees.{Ident, PackageDef, TypeDef}
+import dotty.tools.dotc.ast.untpd.ModuleDef
+import dotty.tools.dotc.core.Contexts.{Context, ContextBase}
+import dotty.tools.dotc.core.StdNames.tpnme
+import dotty.tools.dotc.printing.{PlainPrinter, Printer}
+import dotty.tools.dotc.util.SourceFile
+import dotty.tools.io.PlainFile
+import org.junit.Assert.fail
+import org.junit.Test
+
+class JavaJep445ParserTest extends DottyTest {
+
+  @Test def `parses unnamed class`: Unit = {
+    val code =
+      s"""
+         |// hello
+         |
+         |import some.pkg.*;
+         |
+         |private volatile int x = 0;
+         |@Magic
+         |protected String s = "s";
+         |
+         |void main() {}
+         |""".stripMargin
+
+    val parser =
+      JavaParsers.JavaParser(SourceFile.virtual("MyUnnamed.java", code))
+    val tree = parser.parse()
+
+    println(tree.show)
+
+    fail("TODO")
+  }
+
+  @Test def `treats leading top-level types as nested types of unnamed class`: Unit = {
+    val code =
+      s"""
+         |// hello
+         |
+         |import some.pkg.*;
+         |
+         |interface Inner {}
+         |
+         |static class InnerStatic {}
+         |
+         |void main() {}
+         |""".stripMargin
+
+    val parser =
+      JavaParsers.JavaParser(SourceFile.virtual("MyUnnamed.java", code))
+    val tree = parser.parse()
+
+    println(tree.show)
+
+    fail("TODO")
+  }
+
+  @Test def `treats leading top-level annotated vars as members of unnamed class`: Unit = {
+    val code =
+      s"""
+         |
+         |import some.pkg.*;
+         |
+         |@MyAnnotation
+         |int x = 0;
+         |
+         |void main() {}
+         |""".stripMargin
+
+    val parser =
+      JavaParsers.JavaParser(SourceFile.virtual("MyUnnamed.java", code))
+    val tree = parser.parse()
+
+    println(tree.show)
+
+    fail("TODO")
+  }
+}

tests/pos-java21+/jep445/B.scala

@@ -0,0 +1 @@
+class B

tests/pos-java21+/jep445/UnnamedMainOnly.java

@@ -0,0 +1 @@
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithAnnotatedField.java

@@ -0,0 +1,5 @@
+
+@MyAnnotation
+int myInt = 10;
+
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithField.java

@@ -0,0 +1,9 @@
+private volatile int myInt = 10;
+
+String hello() {
+    return "hello";
+}
+
+interface Inner {}
+
+void main() {}

tests/pos-java21+/jep445/UnnamedStartsWithType.java

@@ -0,0 +1,4 @@
+
+class InnerOfUnnamed {}
+
+void main() {}